1. Field of the Invention
The present invention relates to a method and an apparatus for a quantum communication, and more particularly to a method and an apparatus adapted for use in transmitting a quantum mechanical state or qubit from a sender to a receiver.
2. Description of the Prior Art
Present information processing concerns transmissions conveying classical Boolean states 0 and 1 (bits), in which security of the communication becomes of greatest importance with the development of the network of communication. Therefore, cryptography schemes with a security against undetected eavesdroppers have been proposed.
In contrast to this, recent quantum information processing involves quantum states (qubits; practically, the polarized states of a photon). Quantum information theory concerns the transmission of quantum states from a sender to a receiver. The classical Boolean states 0 and 1 can be represented (encoding) by a pair of different states of a qubit. The qubit can exist in an arbitrary complex linear combination (superposition) of classical Boolean states. In quantum information processing, these arbitrary unknown states can be transmitted by quantum teleportation, which guarantees near-perfect security of the communication.
The original scheme of the teleportation was proposed by C. H. Bennett et al. (Physical Review Letters, vol.70 (1993),1895). According to Bennett, in this scheme, the sender and receiver must prearrange the sharing of an EPR-correlated pair of particles. Sender (Alice) makes a joint measurement on her EPR particle and the unknown quantum system (encoded qubit), and sends to Receiver (Bob) the classical results of four measurement outcomes with equal probabilities via a classical message line. Knowing this, Bob can convert the state of his EPR particle into an exact replica of the unknown state which Alice destroyed. Bob can get the transmitted information through the decoding of this replica.
This is a process that disembodies the exact quantum state of a particle [1] into classical data and Einstein-Podolsky-Rosen (EPR) correlations (an entangled state of particles [2] and [3]), and uses these ingredients to reincarnate the state in another particle [3] which has never been anywhere near the first particle. This disembodiment of the exact quantum state of the particle [1] evades eavesdropping. A quantum teleportation scheme of the type proposed by Bennett et al. is of great interest for quantum communication free from any eavesdropping.
From the measurement theoretical point of view, however, their intact scheme of teleportation would be difficult for its realization, since this original teleportation scheme of an unknown state is based on the existence of non-local long range correlations between the EPR pair of particles relevant to the notion of wave function collapse which is nothing but the almost direct paraphrase of the von Neumann projection postulate. This projection postulate necessarily leads to the EPR paradox. As a result of this paradox, the epistemological discussion on the nature of physical reality cannot be avoided. Such philosophical arguments concerning the conceptual aspects of the EPR problem are beyond the realm of physics. Furthermore, if one follows the von Neumann projection postulate, it is difficult to understand the negative-results measurement and the results of some of the neutron interferometry experiments. A rational theory of measurement and a correct understanding of the EPR problem are necessary to realize quantum teleportation.
It is, therefore, an object of the present invention to provide a new method and apparatus that can realize quantum communication free from any eavesdropping using teleportation without resorting to the von Neumann projection postulate.